Cash held for the bill you didn't plan for.
An emergency fund is money kept liquid and separate, sized to cover three to six months of essential spending — housing, food, insurance, minimum debt payments and transport.
Its job is not return, it is avoiding the credit card. The right comparison is not what the cash earns; it is the APR you would otherwise pay.
It belongs in a high-yield savings account, not in investments that can be down exactly when the emergency arrives.
A $15,000 fund in the national average savings account at 0.38% earns about $57 a year. A high-yield account paying 4% would earn about $600 — same money, same access, different bank.
See the savings page